fighting "amplifier hum" in Scott 800B6
Phil Nelson
After replacing most caps, my Scott 800B6 sounds swell, except for an
annoying background hum. This is not 60-hz power supply hum, but what Ken G.
calls "amplifier hum," i.e., the sort that may result (in my limited
experience) from bad grounding or shielding somewhere, rather than
inadequate filtering of the power supply.
The Scott service manual has five pages of instructions and diagrams
describing changes that were made in the original 800B to reduce hum. I
believe these changes were among the differences which led to the "6" being
added to the model number in the later radios. The fixes mostly involve
moving some grounds and improving the shielding of leads in the audio
circuits, as well as adding another choke in the p-s filter. A small
resistor was also added in the bass control circuit.
The manual also mentions some obvious things, such as subbing the two 6J5
tubes that serve as 1st & 2nd AF amps, subbing the 6SL7, and subbing the two
6L6 final output tubes. Since I have two 800B6s in the shop, this was easy
to do, and produced no change. I even tried swapping speakers, since the
speaker choke performs some p-s filtering. That also produced no change.
The hum is audible even when the volume is turned all the way down. If I
pull the 6SL7 which follows the two 6J5s, the hum disappears completely,
suggesting that the hum is coming from the 6J5/6J5 circuits.
I have replaced some capacitors in the audio circuits, but I tried to be
very careful about lead dress, keeping leads as short as possible and
grounding new caps in the same spot as the old ones. Unfortunately, I can't
remember if this hum was present before I did any recapping. I suspect that
it was. You tend to notice more subtle things about a radio, the longer you
work on it.
Any ideas on how to isolate this problem?
Thanks.
Phil
--Bill M--
>
> I have replaced some capacitors in the audio circuits, but I tried to be
> very careful about lead dress, keeping leads as short as possible and
> grounding new caps in the same spot as the old ones. Unfortunately, I can't
> remember if this hum was present before I did any recapping. I suspect that
> it was. You tend to notice more subtle things about a radio, the longer you
> work on it.
>
> Any ideas on how to isolate this problem?
>
> Thanks.
>
> Phil
Phil,
Not speaking from any personal experience but I have heard it said that
in some cases with new caps the outside foil consideration can become an
issue. Might be worth looking into if you haven't already.
-Bill M
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Phil Nelson
> in some cases with new caps the outside foil consideration can become an
> issue. Might be worth looking into if you haven't already.
Can someone remind me how to tell which end is "outside foil" on a new cap?
I have used a mixture of orange drops and the "little yellow ones" (Xicon?).
P.
Jeffrey D Angus
>
> Can someone remind me how to tell which end is "outside foil" on a new cap?
> I have used a mixture of orange drops and the "little yellow ones" (Xicon?).
Wrap tinfoil around the capacitor. Connect a signal generator to the
tinfoil. Ground one end of the capacitor. Look at the other end with a
scope probe. Reverse the scope and ground connections to the capacitor.
The signal on the scope will be larger in one of the two cases. It is
the larger cases that indicates the outside foil lead.
Think of it as a voltage divider.
A----|(----B----|(----C
The Capacitor between A and B is the actual capacitor.
The Capacitor between B and C is the tin foil and "outside" foil.
Quick and "dirty" method of identifying the outside foil on new caps.
Jeff
--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"
Mark Robinson
One test you can make to help rule out the power supply would be to listen to the hum at the moment you unplug the AC mains.
If the hum stops at that instant, you know the hum is supply related. Before performing the test, hook up an audio source and
get a feel for how long the supply holds up after you remove power. Then try the test with no signal present. By doing this
you will, in effect be running the amp with a pure DC supply, so there will be no ripple on the B+.
The hum may be caused due to poor layout (it sounds like Scott was fighting this due to the number of production changes). The
hum could be due to power supply return current passing near low level stages causing small mv. signals to exist where they
should not. You might have to re-arrange grounds to cure the problem. Also, look for grounds that may be corroded or
oxidized. They might have a somewhat higher resistance that you would expect. You could also try shorting the grids of the
6J5's to ground (near the tube with a short length of wire, to see if the hum is affected. Hope this gives you some ideas.
Mark Robinson
"Phil Nelson" <philn...@antiqueradio.orgNOSPAM> wrote in message
news:8Jdpa.37143$4P1.3...@newsread2.prod.itd.earthlink.net...
Bill Sheppard
I would try the 'signal shorting' technique. That's simply a
small value electrolytic of 10mf or so, with the neg. end connected to a
ground in the suspected area. The postive end is used to short out the
plate of a signal tube, then the grid, then working backward to the
plate of the preceding stage etc., point by point until the hum source
is isolated. If the hum can't be shorted out this way, it is probably
from heater current in the ground system (ground loop), causing a
miniscule AC voltage between ground points. A jumper made of heavy braid
might be useful in isolating such a ground loop problem.
Bill (oc)
Henry 007
"Phil Nelson" <philn...@antiqueradio.orgNOSPAM> wrote in message
news:8Jdpa.37143$4P1.3...@newsread2.prod.itd.earthlink.net...
Try separating the grounds- have cathode grounds wired separately to signal
grounds and power supply grounds, then join the three wires at one point.
Bob
That's the outside foil end.
Eddie Brimer
left as you read the label???
Eddie Brimer
2480 S. Beersheba Rd.
Sharon SC, 29742
visit my web page "THIS OLD RADIO"
http://members.aol.com/EB062559/THISOLDRADIOINDEX.html
Larry W4CSC
minute and see what effect it has on the hum. Short the input plug
while you test this. Set the volume control to zero.
Sometimes we are our own worst enemies with the amp laying out there
on the bench, with the cute flourescent bench light buzzing away
between our eyes and the chassis under test. In my service
truck/shop/stepvan, I'm very fortunate. I took an incandescent desk
light, the cheap kind with the spring-loaded two arm garbage, and
wired it up to my "house battery" 12VDC. I bought a 12V, 50W bulb for
it from Walmart's RV department and it solved 90% of the hum problems
I had when using my favorite antique Navy super-heavy-duty, dual tube
flourescent shop light. When I'm working, I use my fav flourescent.
When I'm testing, I use the DC lighting!
Your hum might be induced right through the open bottom of the
chassis. Sitting it upright on a sheet metal shield to close up the
bottom of the chassis where all the high-Z audio parts are may fix
it.....
Larry W4CSC and other fine old calls since 1957...
--Bill M--
> did i dream it, or did someone tell me once that the outside foil was to the
> left as you read the label???
I've heard that said too but I've also heard it said that it varies too
much to dependably go by.
On some of the new yellow caps for instance, there simply isn't an
outside foil per se because there is an additional foil layer that is
not connected to anything. I've never heard an explanation why they are
made this way.
These outer foil rules have too many thumbs :-)
-Bill
Dan Busetti
I wouldn't waste any time with finding "outside" leads on the caps.
The problem existed before you recapped it and will exist until you
isolate the source(s). Scott knew they had hum problems, that's why
they made all those changes. Make sure the shielded leads are good-
even replace them if you must. The leads from the volume control are
suspect. Hum starts out small. gets amplified by the time it reaches
the last stage.
Ken G.
volume at 0 it is not an issue . Also for those trying to help .. this
set has 5 and 6 foot cables running between the tuner/preamp with all
the controls and the power amp chassis . 2 cables are multi wire covered
with cloth one cable is a single black plastic insulated coax cord that
goes to the amp chassis audio input .
The capacitors in these sets are metal bathtub capacitors much like
millitary stuff has . the rest look like they are full metal covered
axial lead types .05 ,e.t.c.
I cant find any serial numbers on mine to compare .. I also dont notice
any amplifier hum when putting my hand or fingers around any of the
tubes .
WGrills
one of these in 1975 or so, and was quite pleased, except for the slight
residual hum and hiss with volume at zero. After replacing tubes and parts to
no avail, I decided the pre-amp stages downstream of the VC had a bit too much
gain to achieve a decent S/N. My solution was to replace the VC with a
single-shaft dual 1 meg pot. First section connected as per factory, second
section connected at output of 2nd 6J5. This resulted in a dead silent speaker
at zero volume, and much better VC action. In retrospect, I believe the same
results could be achieved by rewiring the single factory control to the output
of the first 6J5 stage. Please note, as per factory, the VC is at the very
input, and everything downstream is running full throttle! I share your
thoughts concerning mods. on old radios, but sometimes a little Texas Pete
improves the steak!
WGrills
output of the first 6J5 would be useful or not: simply lift the .05 coupler
from the 2nd 6J5 grid, tack in a 330K or so resistor from the free end of the
cap to the 2nd grid. If this substantially reduces the noise, moving the VC
connections would be useful (expect to turn the VC a bit higher than before
with this test). Actually, this could be considered a permanent fix if
successful, and VC action is acceptable, without moving VC connections. You
might experiment with resistor value. Also, on shielded cabling, make sure
shields are grounded on only one end. Grounding both ends many times creates
ground loops, and greatly increases hum! Thanks all!
Phil Nelson
plenty loud enough to fill the workshop (OK, a double garage :-), and at
around 1/3 volume it becomes too loud for comfort. If I turned the volume up
to the 1/2-way point, the 15" speaker might walk right off the workbench.
Decreasing the gain a bit wouldn't hurt the radio's BC/FM performance,
although I guess I should do some testing with the auxiliary phono/tv inputs
before I make a permanent commitment. Also, some times you want extra gain
for listening to SW as compared to BC/FM. A nice feature of this
radio/phono/tv combo set is that you can pipe the TV audio through the radio
audio section for "hi-fi" TV sound (better than the dinky 5" TV speaker), so
I definitely want it to work correctly with the TV-to-radio's-audio input.
On the other hand, if I could kill the hum with such a simple change, I'd be
more than happy to walk across the room to twiddle some knobs when I switch
from radio to TV. You need to do that anyway!
Regards,
Phil
P.S. I'm amazed that you can remember any such details from 1975. I can
recall where I lived, and some vague details of who I knew and what happened
that year, but . . . <senior moment!!!> :-)
"WGrills" <wgr...@aol.com> wrote in message
news:20030423011453...@mb-m03.aol.com...
WGrills
Wayne
WGrills
radios! Maybe that's why I had multiple wives, etc., etc.!
WGrills
many hifi mfrs. used this scheme to gain superior S/N ratios. Fisher and
McIntosh come to mind, hence the number of FOUR-GANG volume pots found in the
fifties and sixties. May I also say, the Scott 800B was designed by brilliant
folks, had they not gone out of business, I'm sure a factory fix would have
been forthcoming. I wish I had only an inkling of the expertise of the talented
folks at E.H. Scott!!! Regards, Wayne
WGrills
resistor from first 6J5 grid to ground, connect wire from top of volume control
to first 6J5 grid. Addition of terminal strips to avoid "air terminals"
encouraged.. Lift .05 coupler from grid of 2nd 6J5, now goes to top of volume
control, center tap of VC now goes to 2nd 6J5 grid, finished! Peace, Wayne
Robert Casey
> One or two further thoughts: A quick test to determine if moving the VC to the
> output of the first 6J5 would be useful or not: simply lift the .05 coupler
> from the 2nd 6J5 grid, tack in a 330K or so resistor from the free end of the
> cap to the 2nd grid. If this substantially reduces the noise, moving the VC
> connections would be useful
Maybe paralleling an extra resistor across the plate load resistor would
help. This reduces gain, and is easy to do. Use a resistor of about the
same value as the existing resistor.
Phil Nelson
> Also, on shielded cabling, make sure
> shields are grounded on only one end. Grounding both ends many times
creates
> ground loops, and greatly increases hum!
In fact, that is one of the modifications described in the service manual.
There are long shielded cables for phono and television input. The manual
tells you to eliminate any chassis connection at the terminal end of each
cable.
P.
Ken G.
help. This reduces gain, and is easy to do. Use a resistor of about the
same value as the existing resistor.
Why not just do what WG said and get it fixed ?
--Bill M--
this thing was a hummer but I figure amongst old radio afficianados this
might be a great "project". Free to Ken for that fantastic B&O spkr repair.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2171218322
Phil Nelson
I tacked in a 330K resistor between the coupling cap and the 2nd grid, as
you suggested, and it dramatically reduced the hum. The audio quality is
fine and the set still has more than enough oomph to shake dust off the
shelves around the shop.
I will play around with the value of that resistor before deciding whether
to call this the fix, or go on to move the volume control connections as
described in your other post. One last question on that topic -- when you
say "top side" connection of the volume control, which one do you mean?
Thanks again.
Phil
"WGrills" <wgr...@aol.com> wrote in message
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Ken G.
shields ?
WGrills
as described, that will achieve much better S/N ratio, especially at lower
volumes. Sorry for confusion, I meant "top-side electrically", Of the three
terminals, it would be the outer terminal which ain't grounded!
Phil Nelson
decided to swap all the speakers around and discovered a big difference
among them. Most notable, the speaker from *this* set is not coaxial (hey, I
wasn't paying attention!). It is just a big 15" Jensen speaker with four
leads coming out of the Jones plug from the power/audio chassis. The speaker
from my shop radio, on the other hand, has eight leads coming out of the
Jones plug and has the little spkr mounted in front of the big spkr, etc.
A couple of weeks ago, I tried plugging the "new" speaker (i.e., the one
from the radio/tv/phono console) into my shop radio and didn't notice any
increase in hum from that set. So I concluded that p-s filtering in the
speaker circuit was not an issue.
Today, I hauled the "old" coax speaker off the shelf and plugged it into the
"new" 800B6, and guess what -- it plays with less hum plus (duh) better
audio.
Undaunted, I dusted off the hotwired setup that I created back when I had
bought my shop radio (tuner & audio/power chassis) without a speaker and
needed to sub a PM speaker with resistors in place of the original field
coils. Hooking this up to the "new" 800B6 via a newer hi-fi speaker, I was
treated to wonderful audio with DEAD quiet at zero volume.
If I weren't such a fussbudget, I might nail the new hi-fi speaker with the
hotwired Jones plug assembly into the bottom of this console and pretend
that it's all original. Another choice would be to install the "old" coax
speaker in the console, use the hotwired setup for my shop radio, and sell
the 15" Jensen speaker. Not sure what I could do to get better filtering
using the original (non-coax) 15" speaker -- perhaps install another choke
in line with its field coil?
Regards,
Phil
"WGrills" <wgr...@aol.com> wrote in message
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